Metabolic acidemia has been reported to increase nitrogen catabolism in chronic renal failure. However, in many of these studies the degree of acidemia produced was severe. We gave an acid load to 6 CAPD patients to determine whether mild to moderate acidemia in well dialyzed individuals also causes protein wasting. Patients were studied for 22 days on a constant diet under metabolic conditions in a Clinical Research Center (CRC). Diet, urine, fecal, and dialysate nitrogen were measured daily or in pools throughout the study by chemiluminescence. Patients were administered an equimolar quantity of NaHCO3 and NH4CI for 22 days starting 10 days prior to admission to the CRC. NaHCO3 was discontinued on day 13 and the patients' blood pH decreased from 7.40+0.01SEM to 7.29+1:0.01 (p<0.OO1) from the first to the last day of study. Serum HCO3 fell from 24.8+0.79 to 14.9+1.0 mE-q/L (p<O.OOl) Serum potassium increased from 4.6+O.34 to 5.+0.32 mEq/L (p=0.04). There was a slight increase in serum cortisol measured serially over the 22 days (p~0.05). however, the increase occurred throughout the study. Likewise, SUN increased from 66.3+7.9 to 79.9+6.8 mg/dl (p=0.01). However, the SUN rose throughout the study and the increment in SUN was not different when the first 12 days was compared to the last 10 days. Leucine turnover was measured on days 11 and 22 in 4 patients. Three patients demonstrated increased oxidation (7.2+1.7 vs 10.5+O.5 umol/kg/hr). but oxidation was unchanged in the 4th (p=NS). There was no change in luecine synthesis. The effect of acidemia on N balance was at most, modest. These findings are consistent with the hypothesis that mild to moderate acidemia may promote nitrogen catabolism in some, but not all, well dialyzed CAPD patients.